The transcription factor signal transducer and activator of transcription 3 (STAT3) participates in a wide variety of physiological processes and directs seemingly contradictory responses, such as proliferation and apoptosis. The constitutive activation of STAT3 promotes tumor growth and angiogenesis and is associated with drug resistance in cancer therapy. In contrast, in the heart, the down-regulation of STAT3 has been associated with end-stage heart failure in patients. Moreover, multiple studies showed that the activation of STAT3 promotes cardiomyocyte survival and hypertrophy, as well as cardiac angiogenesis, in response to various pathophysiologic stimuli, strongly suggesting that STAT3 is beneficial for the heart. Conditional knockout (STAT3-KO) mice harboring a cardiomyocyte-restricted deletion of STAT3 showed enhanced susceptibility to cardiac injury caused by myocardial ischemia, systemic inflammation, or drug toxicity. STAT3-KO mice were also more prone to the pathogenesis of age-related heart failure. Thus, STAT3 is involved in multiple mechanisms required for the protection of the heart from injury and heart failure. These observations should be taken into account in designing novel therapeutic strategies for the prevention of cardiac failure.